Washington State Star Party
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Washington State Star Party clear sky chart from 2005-11-06 to 2024-12-24.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	7% 7% 3 6% 4 4 4 3 5% 7% 50%	105715
	Night	8% 9% 3 7% 5% 4 4 4 5% 7% 44%	102972
Transparency	Day	34% 6% 11% 22% 19% 9%	105484
	Night	38% 6% 11% 22% 16% 6%	102849
Seeing	Day	9% 8% 22% 30% 22% 9%	14408
	Night	12% 5% 23% 36% 21% 3	23484

By Month

Percentage Cloud Cover NW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 20% 6% 12% 6% 5% 3 3 4 5% 23%	2539
	11th to 20th	Day	16% 15% 5% 10% 6% 5% 4 4 5% 6% 24%	2516
	21st to end	Day	22% 23% 7% 15% 5% 4 3 2 3 4 12%	2794
February	1st to 10th	Day	11% 14% 4 10% 7% 5% 5% 4 6% 6% 27%	1629
	11th to 20th	Day	9% 13% 4 9% 6% 4 4 3 7% 8% 33%	1683
	21st to end	Day	7% 12% 4 9% 7% 5% 5% 4 6% 9% 33%	1570
March	1st to 10th	Day	6% 8% 3 8% 5% 5% 5% 5% 7% 9% 40%	3138
	11th to 20th	Day	5% 7% 2 5% 3 4 4 4 5% 7% 55%	3335
	21st to end	Day	9% 8% 3 7% 5% 5% 5% 5% 7% 8% 38%	4120
April	1st to 10th	Day	9% 8% 3 7% 6% 4 6% 5% 6% 9% 38%	2280
	11th to 20th	Day	6% 7% 3 6% 5% 5% 5% 6% 6% 8% 43%	2252
	21st to end	Day	5% 7% 3 7% 7% 5% 5% 4 6% 8% 43%	2510
May	1st to 10th	Day	6% 6% 2 5% 3 3 3 3 5% 8% 55%	4420
	11th to 20th	Day	5% 4 2 4 3 3 4 4 6% 8% 58%	4377
	21st to end	Day	8% 7% 3 8% 6% 6% 6% 4 5% 7% 40%	4813
June	1st to 10th	Day	8% 7% 2 6% 5% 4 5% 5% 7% 7% 44%	2660
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 4 6% 8% 49%	2660
	21st to end	Day	4 5% 1 5% 3 3 3 3 5% 8% 60%	2660
July	1st to 10th	Day	1 1 2 2 2 2 2 3 5% 79%	4420
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 3 79%	4420
	21st to end	Day	2 1 2 1 2 2 2 3 4 80%	4790
August	1st to 10th	Day	2 2 1 3 3 2 3 2 4 6% 72%	4401
	11th to 20th	Day	2 2 1 3 3 2 3 3 5% 7% 70%	4167
	21st to end	Day	4 5% 2 5% 3 3 3 3 4 6% 63%	4180
September	1st to 10th	Day	3 2 1 3 2 3 3 2 5% 7% 70%	2280
	11th to 20th	Day	4 4 2 5% 4 4 4 4 6% 7% 55%	2256
	21st to end	Day	6% 4 2 5% 3 4 5% 3 4 6% 57%	2081
October	1st to 10th	Day	4 4 2 4 3 3 3 3 5% 8% 60%	3180
	11th to 20th	Day	6% 6% 2 6% 5% 6% 6% 4 6% 8% 46%	3115
	21st to end	Day	9% 6% 2 6% 4 5% 5% 4 5% 7% 47%	3101
November	1st to 10th	Day	13% 10% 3 9% 6% 5% 5% 4 6% 6% 33%	1745
	11th to 20th	Day	10% 16% 4 9% 6% 5% 5% 3 5% 6% 31%	1600
	21st to end	Day	12% 14% 6% 12% 6% 4 5% 4 4 9% 24%	1600
December	1st to 10th	Day	11% 15% 5% 13% 7% 4 5% 3 5% 7% 26%	1600
	11th to 20th	Day	16% 17% 7% 16% 6% 5% 4 3 5% 5% 15%	2005
	21st to end	Day	11% 21% 6% 14% 7% 6% 5% 4 5% 5% 16%	2818
January	1st to 10th	Night	14% 19% 6% 12% 6% 5% 5% 3 5% 5% 20%	4406
	11th to 20th	Night	15% 15% 4 11% 6% 4 4 3 4 7% 27%	4382
	21st to end	Night	18% 21% 8% 16% 6% 4 4 3 4 4 12%	4862
February	1st to 10th	Night	10% 13% 5% 9% 6% 5% 5% 5% 6% 7% 29%	2551
	11th to 20th	Night	10% 10% 3 8% 5% 4 5% 5% 6% 8% 35%	2383
	21st to end	Night	7% 12% 3 9% 6% 6% 5% 5% 7% 8% 33%	1884
March	1st to 10th	Night	7% 9% 3 6% 5% 5% 5% 4 8% 10% 39%	3736
	11th to 20th	Night	4 7% 2 4 4 3 4 3 4 7% 58%	3630
	21st to end	Night	7% 10% 2 8% 7% 6% 6% 5% 8% 9% 31%	3533
April	1st to 10th	Night	8% 9% 2 8% 6% 4 5% 4 5% 8% 39%	1900
	11th to 20th	Night	6% 9% 3 6% 4 5% 4 5% 6% 6% 46%	1879
	21st to end	Night	6% 6% 3 5% 6% 4 6% 4 6% 9% 46%	1667
May	1st to 10th	Night	4 5% 2 6% 4 4 4 3 5% 7% 56%	2540
	11th to 20th	Night	5% 4 1 3 3 3 4 3 5% 7% 62%	2516
	21st to end	Night	5% 3 1 5% 4 4 5% 4 6% 10% 52%	2762
June	1st to 10th	Night	8% 6% 1 5% 4 4 4 4 7% 7% 49%	1520
	11th to 20th	Night	5% 4 2 6% 4 3 4 5% 6% 8% 53%	1520
	21st to end	Night	3 2 2 5% 4 4 4 3 4 7% 62%	1520
July	1st to 10th	Night	1 1 1 2 2 2 2 2 3 4 82%	2540
	11th to 20th	Night	1 1 1 1 2 2 1 3 3 85%	2540
	21st to end	Night	1 1 1 2 1 2 2 2 4 4 81%	2762
August	1st to 10th	Night	2 3 1 3 3 2 3 2 4 6% 71%	2529
	11th to 20th	Night	2 2 1 4 3 3 2 3 5% 7% 68%	2649
	21st to end	Night	3 4 1 4 3 3 4 3 4 5% 66%	3498
September	1st to 10th	Night	3 2 1 3 2 3 3 3 4 8% 69%	1900
	11th to 20th	Night	4 4 2 6% 4 3 4 3 5% 8% 59%	1924
	21st to end	Night	5% 4 2 4 3 4 4 5% 6% 7% 58%	2095
October	1st to 10th	Night	5% 3 1 3 3 3 5% 4 7% 8% 58%	3800
	11th to 20th	Night	5% 5% 2 5% 5% 5% 5% 5% 6% 8% 49%	3807
	21st to end	Night	10% 7% 3 7% 5% 4 4 4 5% 7% 44%	4457
November	1st to 10th	Night	14% 9% 3 6% 4 5% 5% 4 6% 8% 35%	2550
	11th to 20th	Night	11% 12% 3 9% 6% 4 5% 5% 6% 6% 35%	2800
	21st to end	Night	12% 14% 5% 10% 6% 6% 5% 4 5% 5% 28%	2800
December	1st to 10th	Night	12% 14% 5% 11% 6% 5% 5% 4 6% 7% 27%	2800
	11th to 20th	Night	15% 15% 6% 18% 8% 6% 5% 4 5% 6% 14%	3420
	21st to end	Night	14% 20% 6% 11% 6% 5% 5% 3 5% 6% 20%	4910

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	65% 6% 8% 13% 8% 1	2539
	11th to 20th	Day	61% 6% 14% 15% 3 2	2540
	21st to end	Day	75% 7% 7% 7% 3	2794
February	1st to 10th	Day	56% 9% 15% 15% 5% 1	1629
	11th to 20th	Day	48% 8% 15% 19% 10% 1	1692
	21st to end	Day	47% 9% 14% 19% 10% 1	1570
March	1st to 10th	Day	38% 7% 13% 23% 18% 1	3138
	11th to 20th	Day	28% 5% 13% 43% 11% 1	3335
	21st to end	Day	41% 9% 18% 18% 12% 1	4120
April	1st to 10th	Day	41% 8% 15% 22% 13% 2	2280
	11th to 20th	Day	35% 7% 15% 21% 17% 5%	2252
	21st to end	Day	36% 7% 12% 24% 18% 4	2510
May	1st to 10th	Day	25% 4 11% 22% 31% 7%	4420
	11th to 20th	Day	22% 5% 12% 32% 24% 6%	4380
	21st to end	Day	39% 8% 14% 25% 12% 2	4814
June	1st to 10th	Day	37% 6% 12% 21% 20% 5%	2660
	11th to 20th	Day	30% 6% 11% 27% 20% 5%	2660
	21st to end	Day	22% 4 9% 29% 26% 9%	2660
July	1st to 10th	Day	10% 3 5% 14% 30% 38%	4351
	11th to 20th	Day	12% 2 4 11% 23% 49%	4345
	21st to end	Day	10% 2 5% 18% 35% 29%	4639
August	1st to 10th	Day	14% 3 6% 18% 36% 22%	4384
	11th to 20th	Day	14% 4 9% 36% 30% 7%	4209
	21st to end	Day	23% 4 9% 26% 31% 7%	4179
September	1st to 10th	Day	16% 3 10% 27% 32% 11%	2280
	11th to 20th	Day	26% 7% 12% 23% 25% 8%	2256
	21st to end	Day	27% 6% 16% 29% 17% 6%	2081
October	1st to 10th	Day	23% 5% 12% 37% 21% 2	3180
	11th to 20th	Day	35% 7% 16% 24% 18% 1	3115
	21st to end	Day	36% 7% 13% 26% 14% 4	3101
November	1st to 10th	Day	50% 7% 16% 17% 9% 1	1745
	11th to 20th	Day	52% 9% 13% 17% 8% 1	1600
	21st to end	Day	57% 11% 13% 11% 7% 2	1600
December	1st to 10th	Day	58% 7% 14% 14% 6%	1600
	11th to 20th	Day	72% 7% 7% 7% 5%	2008
	21st to end	Day	69% 9% 9% 9% 5%	2818
January	1st to 10th	Night	65% 7% 10% 12% 5% 1	4406
	11th to 20th	Night	56% 7% 12% 19% 6%	4420
	21st to end	Night	74% 8% 9% 7% 2 1	4862
February	1st to 10th	Night	51% 9% 17% 15% 7% 1	2551
	11th to 20th	Night	45% 8% 18% 18% 10% 1	2385
	21st to end	Night	46% 9% 14% 19% 11% 1	1884
March	1st to 10th	Night	37% 7% 13% 23% 17% 3	3736
	11th to 20th	Night	26% 5% 12% 41% 15% 1	3631
	21st to end	Night	43% 10% 17% 24% 5%	3533
April	1st to 10th	Night	41% 6% 15% 25% 11% 2	1900
	11th to 20th	Night	35% 6% 13% 24% 18% 4	1879
	21st to end	Night	34% 7% 12% 26% 18% 3	1667
May	1st to 10th	Night	27% 4 10% 19% 35% 6%	2540
	11th to 20th	Night	20% 4 10% 31% 32% 3	2516
	21st to end	Night	25% 8% 18% 37% 10% 2	2762
June	1st to 10th	Night	33% 5% 10% 24% 22% 6%	1520
	11th to 20th	Night	27% 7% 12% 29% 21% 4	1520
	21st to end	Night	23% 4 10% 29% 27% 8%	1520
July	1st to 10th	Night	9% 2 5% 15% 29% 40%	2500
	11th to 20th	Night	7% 1 4 12% 23% 52%	2492
	21st to end	Night	8% 2 4 18% 34% 34%	2674
August	1st to 10th	Night	14% 3 5% 17% 36% 25%	2521
	11th to 20th	Night	16% 4 8% 36% 28% 8%	2673
	21st to end	Night	19% 5% 8% 27% 30% 12%	3498
September	1st to 10th	Night	16% 4 8% 26% 32% 15%	1900
	11th to 20th	Night	23% 5% 10% 25% 27% 10%	1924
	21st to end	Night	24% 6% 13% 31% 18% 7%	2095
October	1st to 10th	Night	22% 5% 7% 41% 22% 1	3800
	11th to 20th	Night	32% 7% 13% 27% 19% 2	3807
	21st to end	Night	39% 6% 12% 26% 14% 3	4457
November	1st to 10th	Night	45% 8% 17% 18% 9% 3	2550
	11th to 20th	Night	48% 9% 15% 17% 9% 2	2800
	21st to end	Night	55% 9% 12% 15% 8% 1	2800
December	1st to 10th	Night	55% 9% 13% 18% 5%	2800
	11th to 20th	Night	70% 9% 10% 7% 3	3416
	21st to end	Night	64% 8% 10% 9% 8% 1	4910

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	26% 8% 23% 28% 12% 3	180
	11th to 20th	Day	23% 9% 15% 31% 16% 6%	180
	21st to end	Day	25% 4 15% 35% 16% 6%	198
February	1st to 10th	Day	16% 7% 21% 39% 13% 3	289
	11th to 20th	Day	16% 6% 25% 33% 17% 4	374
	21st to end	Day	10% 6% 28% 31% 21% 4	382
March	1st to 10th	Day	11% 7% 27% 31% 19% 6%	460
	11th to 20th	Day	10% 10% 23% 25% 23% 9%	460
	21st to end	Day	11% 12% 27% 31% 16% 4	506
April	1st to 10th	Day	12% 13% 24% 28% 19% 4	460
	11th to 20th	Day	7% 17% 31% 23% 15% 7%	452
	21st to end	Day	7% 14% 25% 26% 19% 8%	460
May	1st to 10th	Day	7% 12% 23% 35% 19% 5%	463
	11th to 20th	Day	13% 9% 22% 25% 19% 13%	488
	21st to end	Day	9% 8% 22% 24% 24% 13%	529
June	1st to 10th	Day	7% 11% 25% 30% 19% 9%	490
	11th to 20th	Day	6% 12% 29% 30% 17% 5%	490
	21st to end	Day	3 7% 22% 26% 30% 12%	490
July	1st to 10th	Day	4 5% 17% 37% 30% 7%	486
	11th to 20th	Day	2 4 21% 30% 31% 12%	484
	21st to end	Day	1 4 15% 33% 32% 16%	531
August	1st to 10th	Day	3 5% 15% 30% 31% 16%	487
	11th to 20th	Day	1 3 16% 26% 34% 19%	477
	21st to end	Day	4 6% 19% 29% 25% 17%	539
September	1st to 10th	Day	4 5% 16% 31% 28% 17%	490
	11th to 20th	Day	5% 8% 19% 30% 26% 12%	490
	21st to end	Day	8% 11% 19% 31% 23% 8%	490
October	1st to 10th	Day	8% 8% 26% 29% 22% 8%	490
	11th to 20th	Day	9% 7% 21% 28% 27% 8%	469
	21st to end	Day	14% 10% 28% 30% 15% 4	330
November	1st to 10th	Day	16% 11% 21% 32% 18% 1	290
	11th to 20th	Day	13% 17% 25% 25% 17% 6%	200
	21st to end	Day	14% 6% 21% 31% 23% 5%	200
December	1st to 10th	Day	16% 7% 16% 27% 24% 11%	200
	11th to 20th	Day	17% 7% 31% 31% 13% 3	200
	21st to end	Day	19% 8% 19% 36% 15% 4	204
January	1st to 10th	Night	23% 6% 26% 29% 14% 2	760
	11th to 20th	Night	22% 4 22% 31% 19% 3	760
	21st to end	Night	19% 3 20% 38% 17% 2	836
February	1st to 10th	Night	16% 4 26% 38% 14% 2	760
	11th to 20th	Night	14% 5% 25% 35% 19% 2	744
	21st to end	Night	11% 5% 29% 35% 16% 4	628
March	1st to 10th	Night	13% 5% 23% 38% 18% 2	760
	11th to 20th	Night	8% 7% 26% 36% 23% 1	760
	21st to end	Night	11% 6% 29% 38% 15% 3	836
April	1st to 10th	Night	10% 8% 21% 38% 21% 2	760
	11th to 20th	Night	9% 8% 30% 33% 16% 4	751
	21st to end	Night	8% 7% 32% 34% 16% 3	527
May	1st to 10th	Night	6% 6% 28% 38% 20% 2	380
	11th to 20th	Night	10% 6% 20% 35% 26% 4	374
	21st to end	Night	8% 4 18% 39% 27% 4	411
June	1st to 10th	Night	7% 6% 24% 41% 20% 2	380
	11th to 20th	Night	6% 7% 29% 38% 17% 2	380
	21st to end	Night	4 3 27% 36% 28% 2	380
July	1st to 10th	Night	3 3 20% 44% 29% 2	370
	11th to 20th	Night	2 2 24% 42% 29% 1	368
	21st to end	Night	2 2 18% 39% 36% 3	416
August	1st to 10th	Night	2 2 16% 34% 40% 6%	378
	11th to 20th	Night	2 1 13% 35% 39% 9%	463
	21st to end	Night	3 3 17% 40% 32% 5%	836
September	1st to 10th	Night	3 2 19% 37% 32% 6%	760
	11th to 20th	Night	6% 4 18% 29% 35% 8%	760
	21st to end	Night	7% 3 21% 36% 29% 4	760
October	1st to 10th	Night	10% 5% 16% 39% 28% 1	760
	11th to 20th	Night	9% 5% 22% 35% 24% 4	760
	21st to end	Night	15% 4 24% 37% 17% 4	836
November	1st to 10th	Night	18% 6% 20% 42% 13% 1	780
	11th to 20th	Night	16% 10% 27% 31% 14% 3	800
	21st to end	Night	18% 5% 21% 33% 20% 3	800
December	1st to 10th	Night	15% 6% 20% 38% 18% 3	800
	11th to 20th	Night	21% 6% 19% 39% 13% 1	800
	21st to end	Night	22% 5% 23% 38% 10% 1	850

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